Short-circuit is known to be very likely to occur during a-c welding in the electrode gap at the moment of transition of welding current through zero point since electromagnetic forces associated with the flow of welding current cease to act upon a drop of molten metal of the electrode and upon the bath of molten metal of the workpiece being welded. Thus, the drop will hang down under gravity along the electrode axis, and molten metal of the bath, that has been forced out from under the arc beforehands, is straightened to approach the drop.
Such short-circuits may be the reasons for distruption of welding process (especially during power-assisted welding with mechanical supply of wire electrode) since energy supply from the welding transformer is interrupted in this case, and short-circuit may last longer, the unmelted end face of the electrode being sumberged in the bath and touching its bottom. Immediately after that (when maximum short-circuit current value is achieved), the electrode is burnt. This results in a substantial metal outburst from the bath and, not unfrequently, in a disruption of the arc and interruption of welding process.
Known in the art is an apparatus for a-c arc welding (cf. USSR Inventor's Certificate No. 523769, Cl. B 23 K 9/00, publ. 1976), comprising an arc stabilizer which ensures that one pulse of a switch capacitor discharge passes through the electrode gap every moment the welding current passes through zero point. This pulse is capable of preventing short-circuit from occurring at that moment but it cannot reignite the arc. Therefore, such an apparatus can only stabilize the arc in welding with a non-consumable electrode or with manual arc welding with a consumable electrode when short-circuits in the electrode gap at the moment the welding current passes through zero point would not disrupt welding process. For this reason the apparatus cannot be used for power-assisted welding with mechanical supply of the electrode.
The most similar to the invention is an apparatus for a-c arc welding with a consumable electrode with an arc stabilizing feature (cf. USSR Inventor's Certificate No. 603521, Cl. B 23 K 9/06, publ. 1978).
This apparatus comprises a welding transformer having a secondary winding connected to an electrode gap and a pulse generator having a power circuit connected in parallel with the electrode gap and consisting of a series circuit including a thyristor switch, a power supply winding and a switched capacitor, and a thyristor switch control unit having a zero detector connected in parallel with the electrode gap and a control pulse former.
When welding current passes through zero point (with a respective change in polarity at the electrode gap), the zero detector will respond to this change by forming a signal that goes to the control pulse former which will cause one of the thyristors of the switch to fire. The switched capacitor is thus pulse charged through the electrode gap, and the abovementioned switch thyristor is blocked. During the next halfwave of the welding current, when polarity at the electrode gap changes again, the zero detector will generate a signal fed to the control pulse former which will cause a second thyristor of the switch to fire. The switched capacitor will be pulse recharged through the electrode gap. Therefore, upon every passage of welding current through zero point, one pulse will pass through the electrode gap.
In case of a power-assisted welding with a consumable electrode, this pulse is capable of breaking-down liquid metal bridge between the electrode and workpiece, which is formed upon short-circuit, but it cannot reignite the arc. The arc will be reignited after a necessary voltage in the secondary winding of the welding transformer will have been reached.